For example, a device in which a reversible unit is mounted across two adjacently disposed electromagnetic contactors and the reversible unit is mechanically interlocked so as to prevent the two electromagnetic contactors from being closed simultaneously (simultaneous ON operation) is known as a reversible electromagnetic contactor that is connected to the control circuit of an induction motor and performs direct-reverse operation control of the induction motor (for example, Patent Document 1).
FIGS. 11 and 12 illustrate the conventional reversible electromagnetic contactor similar to that described in Patent Document 1. In an electromagnetic contactor 1a shown in FIG. 11, a fixed iron core (not shown in the figure), a movable iron core (not shown in the figure) disposed opposite the fixed iron core, and a coil (not shown in the figure) disposed on the outer circumference of the main leg of the fixed iron core are housed in the lower portion inside a case 5, and when the coil is energized and the movable iron core is attracted to the fixed iron core, a movable contact (not shown in the figure) fixed to a movable contact support 4 and a fixed contact are opened and closed. A display window 3a is formed in an arc-extinguishing cover 2a provided on top of the electromagnetic contactor 1a, and an operation indication piece 4a fixed to the movable contact support 4 protrudes into the display window 3a. 
Another electromagnetic contactor 1b, which is disposed adjacently to the electromagnetic contactor 1a, has the same structure, and when the coil thereof is energized and the movable iron core is attracted to the fixed iron core, a movable contact fixed to a movable contact support 4 and a fixed contact are opened and closed. A display window 3b is formed in an arc-extinguishing cover 2b provided on top of the electromagnetic contactor, and an operation indication piece 4b fixed to the movable contact support (not shown in the figure) protrudes into the display window 3b. 
As shown in FIG. 11, the reversible unit 6 comprises a unit bottom plate 6a connected to arc-extinguishing covers 2a, 2b in a state of being laid across the two electromagnetic contactors 1a, 1b and a unit cover 6b that is engaged by the circumferential edge thereof with the circumferential edge of the unit bottom plate 6a. A first interlock plate 6c, a second interlock plate 6d, and a lock piece 6e constituting a lock mechanism are rotationally connected to each other in the inner space formed by these unit bottom plate 6a and the unit cover 6b. Tubular connection bridges 6f, 6g are formed at the rear surface of the first interlock plate 6c and the second interlock plate 6d at the end side thereof and protrude to the outside through openings 6h, 6i formed in the bottom plate 6a. 
When the connection bridges 6f, 6g of the reversible unit 6 are connected in the respective fitting state thereof to the head portions of the operation indicating pieces 4a, 4b of the two electromagnetic contactors 1a, 1b in a state in which the unit bottom plate 6a abuts on the air-extinguishing covers 2a, 2b, the lock piece 6e causes the rotation of either of the first interlock plate 6c and the second interlock plate 6d, restricts the rotation of the other of the interlock plates, and enables the movement of only one of the operation indication pieces 4a, 4b, thereby performing mechanical interlock such that makes it impossible to close the two electromagnetic contactors 1a, 1b simultaneously.
FIG. 13 shows an example of a control circuit for an induction motor provided with an electrical interlock in addition to the mechanical interlock performed by the reversible unit 6.
In FIG. 13, a first switch-on push-button 11 and a second switch-on push-button 12 are connected in parallel to a push-button 10, and these first and second switch-on push-buttons 11, 12 and auxiliary contacts 13, 14 of normally-closed contacts (contacts (b)) of the two electromagnetic contactors 1a, 1b are connected in series.
Further, a coil C1a and a coil C1b of the two electromagnetic contactors 1a, 1b and the auxiliary contacts 13, 14 of contacts (b) are connected in series, the coil C1a is connected to the auxiliary contact 13 of the contact (b), the coil C1b is connected to the auxiliary contact 14 of the contact (b), and the electrical interlock is performed such that when an exciting circuit of either of the coil C1a and the coil C1b is closed, the exciting circuit of the other of the coil C1a and the coil C1b is open.
The first switch-on push-button 11 and the second switch-on push-button 12 are provided with respective normally-open contacts 11a, 12a and normally-closed contacts 11b, 12b and have the configuration such that the normally-open contact 11a is mechanically interlocked with the normally-closed contact 11b, and the normally-open contact 12a is mechanically interlocked with the normally-closed contact 12b. 
Patent Document 1: Japanese Patent Application Publication No. H3-266325.
As shown in FIG. 13, the normally-closed contacts (contacts (b)) are necessary to ensure electrical interlock. Therefore, in the case of the two electromagnetic contactors 1a, 1b which incorporate only one pole of the auxiliary contacts of the normally-open contacts (contacts (a)), an additional auxiliary contact unit should be connected because the normally-closed contacts (contacts (b)) are necessary. When the auxiliary contact unit is thus connected to ensure electrical interlock, outer dimensions are increased and problems are associated in terms of size reduction. In addition, the device cost can be increased since the auxiliary contact unit is used.